The production of lightfast polyurethane or polyurethane-urea elastomers using aliphatic and/or cycloaliphatic polyisocyanates is known.
There is at present a growing market interest in flexible, light-resistant and weather-resistant polyurethane and polyurethane-urea compositions for a variety of different applications, for example for the production of components for automotive or furniture applications, as sealing compounds for window glass encapsulation for example, or as potting compounds for electronic or optoelectronic components.
The production of elastic lightfast polyurethane or polyurethane-urea elastomers has already been described many times. The aliphatic and/or cycloaliphatic diisocyanates available in industry, such as for example 1,6-diisocyanatohexane (HDI), 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate, IPDI) and/or 2,4′- and/or 4,4′-diisocyanatodicyclohexylmethane (H12-MDI) or oligomeric derivatives of these diisocyanates, are generally used as polyisocyanate components.
For example, WO 1998/014492 describes a reaction injection moulding (RIM) process for the production of optionally foamed, lightfast, flexible or semi-flexible polyurethane elastomers, wherein a crude IPDI trimer which contains monomeric IPDI and can additionally contain up to 10 wt. % of an IPDI prepolymer and optionally up to 5 wt. % of further monomeric diisocyanates is reacted with a combination of polymeric polyether polyols, hydroxy-functional low-molecular-weight chain extenders and an amine-type initiator component in the presence of organometallic compounds as catalysts.
According to the teaching of EP-A 0 275 010, elastic, light-resistant, optionally foamed polyurethane mouldings can also be produced using mixtures of low-monomer HDI trimers or HDI/IPDI mixed trimers containing at least 10 wt. % of monomeric isophorone diisocyanates as the polyisocyanate component and reaction partners which are at least difunctionally reactive to isocyanate groups.
Common to both processes is the disadvantage that they work with large amounts of low-molecular-weight monomeric diisocyanates, which are classed as toxic materials and in some cases exhibit a considerable vapour pressure. For occupational health reasons the processing of these monomeric diisocyanates requires a very high level of safety precautions to be taken. There is also the possibility, particularly if a polyisocyanate excess is used, as proposed for example in EP-A 0 275 010, of unreacted monomeric diisocyanate remaining in the manufactured moulding for some time and slowly evaporating from it.
There has been no shortage of attempts to produce elastic, lightfast polyurethane compositions on the basis of low-monomer, higher-molecular-weight, non-toxic polyisocyanates, in particular those based on the known aliphatic polyisocyanates having a biuret, isocyanurate or uretdione structure.
WO 2007/107210 describes crosslinked flexible polyurethane compositions which are used for example as a silicone substitute in the production of seals. These polyurethanes consist of mixtures of polymeric diols and triols and up to 50 wt. % of special monofunctional compounds as “internal plasticisers”, which are preferably cured with low-monomer, at least trifunctional aliphatic polyisocyanates, for example trimers of HDI. Although the monoalcohols used as internal plasticisers, such as for example N-hydroxyalkyl-substituted aromatic sulfonic acid esters or amides or monofunctional polyether alcohols, give the polyurethane composition a permanent elasticity and prevent embrittlement of the material as they are permanently incorporated by chemical means and cannot migrate out of the moulding, they also considerably reduce the isocyanate functionality and hence the network density, and this has a negative influence inter alia on the resistance to solvents and chemicals.
The same applies to the highly undercured polyurethanes described in EP-A 0 026 123, such as can be obtained by the reaction of polyfunctional polyols with low-monomer polyisocyanates, for example HDI biurets, in an equivalents ratio of isocyanate to hydroxyl groups of 0.2 to 0.6. Such undercured polyurethane compositions are extremely soft, generally tacky and unsuitable for the production of mouldings. They are used as an interlayer or adhesive in the production of multilayer glass, glass/plastic or plastic composite sheets, for safety glass for example.